ONSET AND GROWTH OF CONDUCTION IN POLYIMIDE KAPTON INDUCED BY SWIFT HEAVY-ION IRRADIATION

In order to follow the insulator to conductor transition in a polymer when it is subject to a progressive degradation process, we have irrad iated about 50 samples of poly(imide) Kapton with heavy ions and measu red their conductivities as functions of temperature, electric field, and frequency. Electron spin resonance measurements gave the spin susc eptibility, Linewidth, g factor, and relaxation times along each step of transforming the conductivity of the material. We defined a single damage parameter that characterizes a unique damage scale for the four different ions and the different irradiation energies that we used. F rom the point of view of the electronic properties, the degradation pr ocess consists mainly of the nucleation and growth of carbon disordere d clusters which are rich in pi-conjugated bonds. During the transform ation, the conductivity reflects the competition between two contribut ions, one of which is related to an initial disordered irradiation-dop ed polymeric substrate and the second to a hopping network progressive ly built from the merging of the clusters. The initial substrate contr ibution, visible in the first stage of irradiation, corresponds to a s imple temperature-activated behavior and to a clear Poole-Frenkel depe ndence with the electric field, while the latter shows all the feature s of nonadiabatic hopping in a granular system. Adiabatic hopping with in the clusters invades progressively all the sample in the final stag e.